This invention relates generally to an inertial navigation system (INS), and more specifically, to methods and systems which utilize a true airspeed to improve the vertical accuracy of an INS.
An inertial navigation system (INS), for example, an INS within an aircraft, provides a pilot and other systems within the aircraft with positional data. At least one known INS includes a vertical channel, which provides both a vertical velocity and a vertical position along a vertical axis. The vertical axis is defined as an axis that is perpendicular to the local level plane. Such an INS employs a triad of gyroscopes and accelerometers. These gyroscopes and accelerometers are mounted in an orthogonal manner with respect to one another, either physically or mathematically, such that a center of percussion and a center of rotation coincide as closely as possible.
The gyroscopes provide the ability to measure an angular rate to stabilize the platform (e.g., the aircraft) in inertial space, and the accelerometers provide a measurement of linear acceleration. The angular rate and acceleration information is provided to a navigation computer which, as part of the INS, provides a position, a velocity, and an attitude of the aircraft. The INS provides an independent navigation solution (e.g., the positional data) by measuring the dynamics (based on the signals received from the gyroscopes and accelerometers) of a flight from which such navigation parameters are produced.
However, the vertical channel of these inertial navigation systems are by nature unstable. This instability is caused by errors in the measurement of vertical acceleration. The vertical acceleration is measured utilizing the above described gyroscopes and accelerometers, and then integrated to provide vertical velocity, and then integrated again to provide a position. There are typically errors related to such inertial instruments, for example, manufacturing flaws, electronic noise, temperature sensitivities, and other sources of error. These errors are compounded as the navigation computer performs the calculations which result in the determination of the vertical acceleration, velocity, and position.
It is known to utilize a control circuit in conjunction with the vertical channel of the INS. The control circuit utilizes data from a barometric altimeter, which is common to many aircraft. The barometric altimeter provides an independent measurement of vertical position which can be used to constrain the position errors from the navigation computer and improve accuracy of the computed Vertical velocity. However, compensation utilizing barometric altimeter data has drawbacks, one of which is that computations originating from the barometric altimeter have a latency of several seconds.